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PHYSICS: C. BARUS 
of mirrors have been drawn and where the angle of rotation a has placed 
mPn into the position m'Pn\ The result is that a part y of the ray d 
is cut off on the left side and a part x added to the ray on the right 
side, so that the path difference which may be assumed to have been 
zero originally is now appreciably incremented, but not symmetrically 
for both sides. 
It may be shown however that the rays n'P'2.T2 and m'PiTi still enter 
the telescope in parallel and that therefore the conditions of interference 
have not been disturbed. This is the interesting feature of the method. 
It will contribute to a more adaptable design of the apparatus for 
general interferometry, if the ray Sn" may also be reversed by reflec- 
tion parallel to itself from a normal mirror allowing a small lateral 
offset, similar on both sides for clearance of the mirrors. Again half 
silvers may be used at m and n for transmission and reflection, which 
method is probably best. These details will here be disregarded. If 
small angles are to be measured the direct ray method is enormously 
more sensitive. 
3. Equations. — To derive the equations certain intercepts of the 
rays figure 2 in addition to x and y, may be defined. Pi P2 is the trace 
of the vertical plane of symmetry of the right angled prism, if rotated 
at an angle a to the right. In this case the reflected ray n'Piq on the 
